1. Field of the Description
The present invention relates, in general, to improved techniques for controlling humanoid robots and other rigid-body dynamical systems, and, more particularly, to operational space control methods and operational space controllers that provide enhanced control over rigid-body dynamical systems such as robotic devices with constraints or constrained movements and such as underactuated robotic devices.
2. Relevant Background
Robots or robotic devices are widely used in manufacturing, assembly, packing and packaging, earth and space exploration, surgery, laboratory research, and entertainment. Some robots may include a series of rigid links or bodies linked together by joints, and some or all of the links may be moved or pivoted about the joints by actuators. Actuators are like the muscles of a robot as they respond to control signals, such as from a control system or controller, to convert energy into movement. The majority of robots use electric motors (DC or AC motors) as actuators to spin a wheel or gear while some actuators are provided in the form of linear actuators or other types of actuators. Robots that work in the real world typically provide some way to manipulate objects (e.g., pick up, move, or otherwise have an effect). For example, a hand (or foot) of a robot may be referred to as an end effector while the arm (or leg) is referred to as a manipulator, and these systems or devices of the robot may be made up of a number of rigid links or members interconnected by joints with movement at the joints controlled by actuators.
An ongoing challenge for those in the field of robotics is how to best or better control a robot or a rigid-body dynamical system to achieve a particular task (e.g., manipulate an item with an end effector). This problem may be greatly complicated when the robot is underactuated. An underactuated robotic system is a system with fewer independent control inputs than the degrees of freedom (DOFs). For example, a robotic arm may have 3 DOFs but the controller may only be able to provide two control inputs or control signals, e.g., the arm may have three joints but only two actuators such that one of the joints is a passive joint while the other two are active or actuated joints. As a result, the input control signals to the actuated joints have to compensate for the passive joint to achieve a desired movement.
Generally, the dynamics of a robot are considered in order to plan and execute fast, dexterous, and compliant motion. More specifically, the inertial and energy characteristics of the end effector or operational DOFs are important for the ultimate success of the task. With this in mind, researchers developed an operational space formulation that provides an equation of motion for a robot by expressing the dynamics in the robot's task space (i.e., the space in which the robot is commanded to operate such as the space of positions and orientations of an end effector). Such an operational space formulation can be used to derive end effector dynamics for rigid-body robot manipulators. The resulting control solutions compenate for (or linearize) operational space dynamics while decoupling tasks from redundant null space dynamics (with “null space” being robot movements not associated with the task space). In these operational space formulations, any additional forces applied within the null space remain dynamically consistent with the tasks.
While providing some improvement in control over robotic systems system as classical manipulators, operational space formulations have not yet been widely applied to a wider range of robotic systems. For example, it may be desirable to apply an operational space formulation as a control solution for modem humanoid robots that are typically complex, high DOF systems in which task dynamics need to be considered for manipulation, balance, locomotion, and so on. Adding to their complexity, humanoid systems may be both underactuated and constrained. The complete representation of their dynamics may have to consider the 6 DOF floating base link connected passively to an inertial reference frame. Additionally, contacts with the environment (e.g., feet, hands, and so on) input constraint forces into the system. Hence, an ongoing challenge for robotics designers is to more fully understand how underactuation and constraints interact with the dynamics of the tasks that a controller is designed to control.
Prior research has attempted to address operational space control within constrained environments. In one formulation, constraints applied at the end effector are considered as an additional task to be controlled, e.g., to realize a desired contact force. In another formulation, systems with more complex kinematic structures were examined including internal closed kinematic chains while others have taken a more general approach that included systems with holonomic constraints applying it to the operational space control of a parallel mechanism.
While this research represents progress toward improved control over robots including underactuated and constrained devices, there remains a need for further improved controllers for robots including rigid-body dynamical systems. Preferably, such controllers and control methods would utilize operational space control.